1. Field of the Invention
The present invention relates to an optical device. More particularly, the present invention relates to a filter device.
2. Description of the Related Art
The illumination system of most projection apparatus is suitable for providing a light beam. After the light beam has passed through a filter device (for example, a color wheel), the filtered light beam is projected on a display device through a series of focusing lenses. Then, the display device transforms the light beam into an image. Finally, the image is projected onto a display screen through a projection lens.
A conventional filter device includes a red filter unit, a blue filter unit and a green filter unit. These filter units are used for filtering incident white light into red light, green light and blue light. As shown in FIG. 1, the points R, G, B corresponding to the red light, the green light and the blue light in the CIE (International Commission on Illumination) 1931 chromaticity diagram form a triangular area. By mixing the red light, the green light, and the blue light, various colors light beams within the triangular area are produced so that the projection apparatus projects color images.
In the conventional technique, the filter device further includes a yellow filter unit, a magenta filter unit and a cyan filter unit in addition to the red filter unit, the blue filter unit and the green filter unit for enhancing the color saturation of the image. Aside from generating red light, green light and blue light, the filter device also produces yellow light, magenta light and cyan light.
Although mixing the aforementioned six colors light increases the color saturation of the image, the points Y, M, C that correspond to the yellow light, the magenta light and the cyan light in the CIE 1931 chromaticity diagram deviate from the triangle area formed by the three points R, G and B. As a result, the gray scale of the image is not smooth, which deteriorates the image quality produced by the projection apparatus. Therefore, the demand for high color saturation in the projection apparatus is not met.